1. Field of the Invention
This invention relates to a method for removing curling of a circuit printable flexible substrate.
More particularly, this invention relates to a method for mechanically removing curling occurring on a circuit printable flexible substrate comprising a metal foil and a film layer of an aromatic polyamideimide or polyimide provided thereon. Further, this invention relates to a method appropriately employable for mechanically and continuously removing curling occurring on a circuit printable continuous flexible substrate comprising a metal foil and a film layer of an aromatic polyamideimide or polyimide provided thereon.
2. Description of Prior Arts
A circuit printable flexible substrate is a substrate comprising a metal foil such as a copper foil and a resin film provided on the metal foil. Such a circuit printable flexible substrate has been recently widely utilized as a wiring substrate for preparing a flexible printed circuit so as to obtain a simplified or highly integrated electronic circuit. Particularly, a circuit printable flexible substrate comprising a metal foil and a film layer of an aromatic polyamideimide or polyimide provided thereon has been mainly utilized because the aromatic polyamideimide and polyimide have excellent properties as a material for forming a resin film of the substrate.
A circuit printable flexible substrate comprising a metal foil and a resin film of the above-mentioned aromatic polyamideimide or polyimide can be manufactured by the following methods:
(1) A method comprising combining a metal foil and a film of an aromatic polyamideimide or polyimide with an adhesive under pressure and heating, and PA1 (2) A method wherein a substrate comprising a metal foil and an aromatic polyamideimide or polyimide is manufactured without using an adhesive. More specifically, this method can be carried out by coating on the said metal foil a solution of an aromatic polyamideimide, an aromatic polyimide precursor or an aromatic polyimide, and then drying the coated solution layer to to cause it to set. PA1 a method wherein a curled substrate is heated to a high temperature for a long time, thereby removing the distortion, as disclosed in Japanese Patent Provisional Publication No. 54(1979)-66966; PA1 a method wherein a curled substrate is wound around a cylinder having a diameter of 70-1,000 mm, placing the surface opposite to the curled face (that is the surface of a metal foil) inside, and then allowing it to stand for a long time at a high temperature, thereby removing the curling, as disclosed in Japanese Patent Provisional Publications No. 54(1979)-108272, No. 54(1979)-111673 and No. 55(1980)-72095; and PA1 a method wherein a curled substrate is wound around a cylinder in the same manner as described above and is allowed to stand in an organic solvent at a high temperature for a long time, thereby removing the curling, as disclosed in Japanese Patent Provisional Publications No. 55(1980)-160489 and No. 56(1981)-23791. PA1 a stage of sliding under tension the curled circuit printable continuous flexible substrate in the longitudinal direction, on a curved surface of a first bar having a curvature radius of 0.5-25 mm which is so arranged as to make an angle of 20.degree.-70.degree. against the width direction of the above continuous substrate supplied and at which the substrate is turned back at a turning angle of not less than 90.degree., placing the metal foil layer in contact with the first bar, at a temperature of not higher than 80.degree. C.; and
The above-mentioned method (1) has been generally employed. In this method, an adhesive layer of 10-30 .mu.m thick is provided between the metal foil and the aromatic polyamideimide or polyimide film. However, it is known that resins generally used for forming the adhesive layer are noticeably inferior to the aromatic polyamideimide and polyimide in such properties as heat resistance, electric property, resistance to chemicals, and mechanical property. Accordingly, the properties of a circuit printable flexible substrate prepared in the method (1) are necessarily defined by the properties of the adhesive resin. For this reason, advantageous features brought about by using the film material of an aromatic polyamideimide or polyimide having excellent properties for the preparation of the insulating layer are not made the best use of.
On the other hand, the method (2) is advantageously simple, and there is no need of utilizing an adhesive layer. Accordingly, the excellent properties of the aromatic polyamideimide or polyimide also become the advantageous properties of the product substrate. A further advantage is that the adhesion between the metal foil and the aromatic polyamideimide or polyimide film layer is not deteriorated at high temperatures. However, a circuit printable flexible substrate manufactured according to this method (2) has a problem of occurrence of curling caused by shrinkage of the volume of the resin film layer. More specifically, in the stage for drying a resin layer of an aromatic polyamideimide or polyimide solution coated on a metal foil, the volume shrinkage of the film layer takes place as a result of evaporation of the solvent, and this volume shrinkage brings about the unfavorable curling. Particularly, in the method wherein a solution of an aromatic polyimide precursor such as an aromatic polyamic acid is coated on a metal foil and heated to give a polyimide film layer through evaporation of the solvent and ring formation of the polyamic acid, both shrinkage by evaporation of the solvent and the volume shrinkable by the ring formation reaction take place, thereby bringing about heavy curling. Furthermore, it is known that the difference of linear expansion coefficient between the resinous material forming the film layer and the metal foil accelerates the occurrence of curling.
The occurrence of such curling is fatally defective for a circuit printable flexible substrate, because such curling causes inconvenience in the course of a screen printing process, a chemical etching process and the like to the substrate. Futher, the so produced curling sometimes causes formation of cracks on a resist and problems such as cutting and formation of short circuit of a conductor.
Accordingly, there have been proposed many improvements for preventing or reducing occurrence of the curling.
For example, as a method for preventing occurrence of curling in the case where an aromatic polyimide film layer which is most suitable for a circuit printable flexible substrate is formed on a metal foil, there has been proposed a method employing a partially ring-closed aromatic polyimide precursor so as to decrease the volume shrinkage, as well as a method wherein the volume shrinkage caused by the ring closing reaction is prevented by coating an aromatic polyimide of a solvent-soluble type. However, even in these improved methods, there still remains a problem of curling occurring in the stage for evaporating the solvent, as well as occurring due to the difference in the linear expansion coefficient between the resinous material and the metal foil. In the case where an aromatic polyamideimide film is formed on a metal foil, the volume shrinkage produced by ring closing reaction hardly occurs, while the curling caused in the stage of evaporation of the solvent and the difference in the linear exapantion coefficient between the metal foil and the resinous material still occurs.
Alternatively, a method for removing curling occurring on a manufactured substrate is known.
Examples of such methods include:
However, since these methods all require a heat treatment at a high temperature for a long time, they are not suitable for removing curling particularly occurring on a circuit printable continuous flexible substrate. Further, a substrate processed by the method comprising a heat treatment in an organic solvent sometimes contains, undesirably, a solvent introduced in the processing.
Methods for removing curling occurring specifically on a circuit printable continuous flexible substrate have been known. As an example, there can be cited a method wherein a metal foil of a curled substrate is bent while heated around a dryer having a curved surface suce as a drum dryer, and then drawn or rolled in the bended direction (MD direction), thereby decreasing curling, as disclosed in Japanese Patent Provisional Publication No. 54(1979)-31480. According to this method, however, since the substrate is drawn or rolled at such a high temperature so as to soften the resin layer, for example, at a high temperature in the vicinity of the glass transition temperature or higher than 100.degree. C. in the presence of a solvent, both the resin film layer and metal foil suffer plastic deformation. Accordingly, this method is defective in that the process is complicated, a pin hole is apt to be produced on a resin film layer, and that curling is apt to reoccur after completion of the curling removal processing, as a result of the difference in linear expansion coefficient between the resin film layer and the metal foil, or spontaneous removal of the remaining solvent.